a. Field of the Invention
The present invention relates to an amplifying system, and more particularly it pertains to an amplifying system whose over-all gain is adjustable to obtain an output signal of the rated level at a good S/N ratio without any distortion caused by the saturation of the amplifier employed therein and noise generated in this amplifier, irrespective of the level of the inputted signal.
B. Brief Description of the Prior Art
An amplifying system for amplifying an electrical signal is often required to be adjustable of its over-all gain to obtain an output having a certain desired level (i.e. rated output level) irrespective of the level of the signal inputted thereto. In this specification, the term "level" means the magnitude standard of the signal and not the instantaneously varying magnitude of the signal. For instance, a pre-amplifier employed in a stereo set is assigned to amplify signals of various levels fed from such signal sources as a disk record player, a tape recorder, an FM-receiver, and so on. In other words, the pre-amplifier is used to amplify such input signal of different levels up to one having a particular level when derived at the output terminal, irrespective of the input-signal level, i.e. irrespective of the type of the signal source connected to the pre-amplifier. Thus, the pre-amplifier is designed to carry out an adjustment of its over-all gain.
To accomplish an over-all gain-adjustment of an amplifying system, therefore, there have been proposed and placed on the market an amplifying system arrangement as shown in FIGS. 1 and 2.
The known amplifying system shown in FIG. 1 is composed of an amplifier A having a certain gain and a potentiometer VR provided at the input side of the amplifier A for adjusting the level of the signal inputted to the amplifier A to adjust the over-all gain of this amplifying system. Needless to say, the over-all gain of this amplifying system can be adjusted, by operating the slider tap of the potentiometer VR, so that there can be obtained an output signal of a desired level at the output terminal OUT, irrespective of the level of the input signal fed to the input terminal IN. For instance, when a signal of a high level is inputted to this amplifying system at its input IN, the slider tap of the potentiometer VR is operated to move downwardly as viewed in FIG. 1 to decrease the over-all gain of the amplifying system.
However, the decreasing operation of the over-all gain of this amplifying system by the operation of the potentiometer VR quite often results in a worsening of the S/N ratio (signal to noise ratio) of the amplifying system for the following reasons. The S/N ratio represents the ratio of the level of the input signal component contained in the output signal of the system to that of the noise component contained in the output signal. The level of the input signal component is proportional to the level of the signal inputted to the amplifier A, while the level of the noise component is dependent mainly upon the level of the noise generated within the amplifier A per se, i.e. the level of the internal noise peculiar to this amplifier A, and this noise level is substantially constant irrespective of and irrelevant to the level of the signal inputted to the amplifier A. Therefore, when the level of the signal inputted to the amplifier A is decreased to a lower level by the operation of the potentiometer VR, there is certainly obtained a lowered over-all gain of the amplifying system, but this is accompanied by an unwanted poor S/N ratio.
FIG. 2 shows the typical arrangement of another amplifying system of the prior art. As is seen in this figure, there is provided, at the output side of an amplifier A having a certain gain, a potentiometer VR intended for adjusting the level of the signal derived at the output terminal OUT which, in turn, is connected to the slider tap, of this potentimeter VR to perform an adjustment of the over-all gain of this amplifying system. By moving this slider tap of the potentiometer VR, it is possible to adjust the over-all gain so that there is delivered at the output terminal OUT an output signal of a desired level, irrespective of and irrelevant to the level of the signal inputted to the input terminal IN of the amplifying system, i.e. in this arrangement, to the input terminal of the amplifier A itself. For example, in the case wherein the input signal of a high level is applied to the input terminal IN, the slider tap of the potentiometer is operated to move downwardly as viewed in FIG. 2 in order to decrease the over-all gain of the whole amplifying system so that the output signal of a desired level is derived at the output terminal OUT. Thus, this known arrangement of FIG. 2 is capable of performing an adjustment of the over-all gain or an adjustment of the output level of the amplifying system.
It should be noted, however, that this conventional arrangement of FIG. 2 has the disadvantage that, in case the input signal of an excessively high level which exceeds the permissible maximum input-level set for its amplifier is inputted to the amplifying system, such a high-level input signal will develop a saturation phenomenon of the amplifier and, as a result, the output signal derived will be an unpleasantly distorted one.
As discussed above, these known amplifying systems are generally arranged to be operative so that the over-all gain is adjusted by a single potentiometer provided either at the input side or at the output side of an amplifier which is a member constituting such amplifying system in cooperation with the potentiometer. In other words, there is provided a single potentiometer as signal-level-variable attenuating means assigned for attenuating the level of the input or output signal of the amplifier to a desired level.